The present invention relates to boride thin films and methods of their formation and, in particular, to magnesium diboride thin films for use in superconducting electronics such as superconducting integrated circuits, in coated-conductor tapes, and other applications using superconductor thin films.
Integrated circuits using superconductors are more suitable for ultrafast processing of digital information than semiconductor-based circuits. Niobium (Nb) based superconductor integrated circuits using rapid single flux quantum (RSFQ) logic have demonstrated the potential to operate at clock frequencies beyond 700 GHz. However, the Nb-based circuits must operate at temperatures close to 4.2 Kelvin (K), which requires heavy cryocoolers with several kilowatts of input power, which is not acceptable for most electronic applications. Circuits based on high temperature superconductors (HTS) would advance the field, but 17 years after their discovery, reproducible HTS Josephson junctions with sufficiently small variations in device parameters have proved elusive.
The success in HTS Josephson junctions has been very limited due to the short coherence length, about 1 nm, in the HTS materials.
The newly-discovered superconductor material magnesium diboride (MgB2) holds great promise for superconducting electronics, in part, because of its relatively high transition temperature (Tc), at which temperature the respective material becomes superconducting and changes in electrical resistance from a finite value to zero. This temperature for MgB2, in bulk, can be as high as 39 K. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor with a relatively long coherence length, about 5 nm. These properties make the prospect of fabricating reproducible uniform Josephson junctions more favorable for MgB2 than for other high temperature superconductors. A MgB2-based circuit can operate at about 25 K, achievable by a compact cryocooler with roughly one-tenth the mass and the power consumption of a 4.2 K cooler of the same cooling capacity. Furthermore, since the ultimate limit on device and circuit speed depends on the product of the junction critical current, Ic, and the junction normal-state resistance, Rn, and since IcRn, is proportional to the energy gap of the superconductor, the larger energy gap in MgB2 could lead to even higher speeds (at very high values of critical current density) than in Nb-based superconductor integrated circuits.
A problem encountered in depositing MgB2 thin films, however, is that a very high Mg vapor pressure is necessary for the thermodynamic stability of the MgB2 phase at elevated temperatures. There are three types of techniques currently used for MgB2 film deposition: 1) Co-evaporation of Mg and B in high vacuum below 320xc2x0 C.; 2) Low temperature deposition of Mgxe2x80x94B or Mgxe2x80x94MgB2 mixture by pulsed laser deposition (PLD) or thermal evaporation, followed by an in situ annealing in the growth chamber; 3) Ex situ annealing of B films, made by PLD or sputtering, at 900xc2x0 C. under a Mg vapor. The films deposited by these technologies either have reduced Tc, poor crystallinity, and/or require an additional step of ex situ annealing in Mg vapor. Further, the surfaces of these films are rough and non-stoichiometric, all of which is undesirable for Josephson junction devices.
Accordingly, a continuing need exists for the efficient manufacture of thin films of superconductors, in particular boride thin films in high throughput and purity.
Advantages of the present invention are thin film materials and methods for their manufacture.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a method which combines physical vapor deposition with chemical vapor deposition. The method comprises introducing or providing a substrate, i.e., a substrate typically used in fabricating semiconductors, into a chamber. The method further includes physically generating vapor from at least one source material, which is within the chamber with the substrate. The vapor of the source material can be physically generated by, for example, heating the source material, ablating the source material, or by employing a pulsed laser upon the source material thereby physically generating vapor of the source material in the chamber. Advantageously, the source material is in close proximity to the substrate to facilitate formation of the thin film.
The method additionally includes introducing at least one precursor to the chamber. The precursors combined with the vapor from the at least one source material to form a thin film of the combined precursor and source material on the substrate. Typically, the vapor from the precursor material and source material combine by chemical reaction to form a thin film that comprises both constituents of the source material and constituents of the precursor. However, physical combinations of the two are also contemplated.
Embodiments of practicing the present invention include physically generating vapor from a source material including magnesium, calcium, titanium or alloys thereof; introducing a boron containing precursor to the chamber; introducing a carrier gas; e.g., hydrogen and/or nitrogen; and forming a boride metal film on the substrate.
Another aspect of the present invention is a thin film suitable for use in semiconductor applications. In an embodiment of the present invention, a magnesium diboride film having high purity, low roughness, and high critical temperature is combined with a substrate.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description wherein embodiments of the present invention are described simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.